What makes Holden feel not depressed anymore?

Holden is heading off to Broadway to buy a record for Phoebe. On the way there, he comes across a young family coming out of church. The mother and father are completely oblivious as their little boy sings, "If a body catch a body coming through the rye." At least that's what Holden thinks he's singing; the word "catch" doesn't appear in the original. In any case, hearing the boy sing makes Holden feel a little less depressed.
The words of the song conjure up an image of Holden standing on the edge of a cliff. All around him kids are playing in the rye, and it's Holden's duty to jump out and catch the kids to stop them from falling over the cliff edge. This fantasy gives Holden a sense of purpose, something sorely lacking in his everyday life. It's not surprising that such a happy thought should lift Holden's mood just a little.